An enameled product is made by forming a vitreous enamel layer on the surface of a substrate consisting of a metal such as steel, cast iron, aluminium, copper or stainless steel, and is manufactured by, for example, forming the metal substrate into a desired shape, applying a glaze (frit) to its surface, and firing the result at a high temperature. Enameled products are resistant to scratching, permit ready removal of oil stains and the like, and are excellent in heat resistant, resistance to acids, and resistance to alkalis. They are therefore used in a wide range of applications, including kitchen equipment, tableware, sanitation containers, and interior and exterior building materials.
Steel sheet enameling ordinarily involves preprocessing (degreasing, pickling and plating with Ni, Co or the like), followed by so-called two-coat enameling in which a glaze ground coat is formed first and then a cover coat is formed. Thanks to advances in steel sheet and enameling technologies, a one-coat method that omits the ground coat has also come into practical use in recent years.
The preprocessing has been a major obstacle to cost reduction owing to the increasing cost of effluent processing and related equipment, coming on top of the equipment, chemical solution, energy and other running costs.
A process that reduces preprocessing to only degreasing and conducts glazing by electrostatic coating is in use but practical application is limited to two-coat enameling that, for ensuring adequate adhesion, requires a ground coat containing an element with adhesion enhancing effect such as Ni, Co or Mo.
Japanese Patent Publication (B) No. S36-19385 and Japanese Patent Publication (A) No. S63-195284, for example, teach technologies that eliminate the need for preprocessing by forming an oxide film on the steel sheet. However, they do not achieve sufficient adhesion between the steel sheet and the enamel layer and are also unsatisfactory in bubble/black spot defect resistance and fishscale resistance. The prior art taught by Japanese Patent Publication (B) No. S36-19385 was developed for application to the relatively easy-to-enamel capped steel of the days before continuous casting and cannot be applied to current steels which are difficult to enamel because nearly all are produced by continuous casting. Later, processes were invented that called for soaking the steel sheet in an Ni solution after forming it with an oxide film (e.g., Japanese Patent Publication (A) No. S63-293173) and, as an improved technique, for coating with an anticorrosive oil (e.g., Japanese Patent Publication (A) No. H1-316470), but adhesion, bubble/black spot defect resistance and fishscale resistance remained inadequate and unsatisfactory. As set out in Japanese Patent Publication (A) No. S63-18086, for example, techniques were also introduced that aimed at achieving uniform glaze application by carrying out roughness control so as to produce an anchoring effect, applying an anticorrosive oil, and using the oil decomposition gas generated during firing to buoy up the glaze. However, it was found difficult to consistently realize adhesion, bubble/black spot defect resistance and fishscale resistance on a par with that when conducting preprocessing.
Moreover, Japanese Patent Publication (A) No. S53-108023 discloses a technique directed to eliminating the need for preprocessing by heating the steel sheet at a relatively low temperature (450-580° C.) to remove oil and adhering a glazing agent composed of an oxide of manganese, molybdenum, cobalt, nickel or the like. This technique is premised on the assumption that sandblasting is performed to ensure good adhesion. However, the reference to “adhering a glazing agent composed of an oxide of manganese, molybdenum, cobalt, nickel or the like” means it is a two-coat enameling technique requiring a ground coat and that it is incapable of achieving one-coat enameling without preprocessing.